Do you have a version of our US www.meetup.com over there? Groups that get together for a specific purpose or inclination, such as speaking French.
Your best bet would be full immersion, hanging around French speakers as much as possible.
Post your discussion in our Germany and France groups here too.
Start by getting some realistic expectations
Thank you, i will try to find there a group.
I have to high expectattions to myself, but if i dont have them i dont learn regularly.
I don't think it is possible to get up to B2 level from complete beginner in just four weeks.
That being said, I agree with Windlord that full immersion in France coupled with lessons would be your best bet, if that suits your learning style not to mention budget. If you want to learn on your own, I recommend the the book "Sprachenlernen leichtgemacht!: Die Birkenbihl-Methode Fremdsprachen zu lernen" (in German) in addition to a self-study course (FSI has free ones, which are dated, but pretty solid at http://www.fsi-language-courses.org/Content.php?page=French)
I took French in high school, but only really got fluent later on by using an abridged version of the Birkenbihl method. I'm now at C1 level, but that took me quite a few years to achieve.